Recently, hydrofluorocarbon(s) (HFC) having the ozone depletion potential of 0, and lower global-warming potential (GWP) has been used as a refrigerant for a refrigerator and the like. HFC is a stable refrigerant compared to chlorofluorocarbon(s) (CFC) and hydrochlorofluorocarbon(s) (HCFC), and does not largely affect to lubricant oil, an organic material, and a metal. On the other hand, as HFC does not have sufficient lubricity, the heat generated at a sliding portion accelerates thermal and/or oxidative degradation of refrigerant oil. Therefore, refrigerant oil having high thermal and chemical stability is desired (NPL 1).
And a refrigerant oil is typically circulated together with a refrigerant through part of the refrigeration cycle, and thus the refrigerant oil is exposed to a high temperature range and a low temperature range. In the low temperature range, particularly, part of the refrigerant oil discharged from a compressor may be retained. If the refrigerant oil is exposed in the low temperature range for a long period, the refrigerant oil is crystallized to thereby reduce a circulating amount of the refrigerant in the refrigeration cycle, leading to a cooling failure. Accordingly, it is extremely important to develop refrigerant oil, which is highly stable without precipitating over a long period even in the low temperature range, in view of reliability of a refrigeration device (PTL 1).
PTL 2 describes heat resistance of refrigerant oil containing hexaester which is made by reacting dipentaerythritol with 3,5,5-trimethylhexanoic acid, as a main component. However, the hexaester does not realize low temperature properties, and does not achieve well balanced characteristics required for industrial lubricating oil, such as refrigerant oil.
NPL 2 discloses hexaester which is made by reacting bispentaerythritol monoformal with acetic acid, as an intermediate of bispentaerythritol monoformal, but does not teach or suggest use of the hexaester as industrial lubricating oil, nor low temperature properties or oxidation stability thereof.